1. Field of the Invention
The present invention relates to a manufacturing method of a thin film transistor and a manufacturing method of a liquid crystal display device.
In this specification, a semiconductor device means all types of devices that can function by utilizing semiconductor characteristics, and a semiconductor circuit, a memory device, an imaging device, a display device, an electro-optical device, an electronic device, and the like are all semiconductor devices.
2. Description of the Related Art
In recent years, thin film transistors that are formed using a semiconductor thin film having a thickness of approximately several nanometers to several hundreds of nanometers over a substrate having an insulating surface such as a glass substrate have been attracting attentions. Thin film transistors are widely used for electronic devices such as ICs (integrated circuits) and electro-optical devices. In particular, thin film transistors are urgently developed as switching elements of image display devices typified by liquid crystal display devices and the like. In an active matrix liquid crystal display device, a voltage is applied between a pixel electrode connected to a selected switching element and an opposite electrode corresponding to the pixel electrode, and thus, a liquid crystal layer disposed between the pixel electrode and the opposite electrode is modulated optically. The optical modulation can be recognized as a display pattern by an observer. An active matrix liquid crystal display device here means a liquid crystal display device which employs a method in which a display pattern is formed on a screen by driving pixel electrodes arranged in matrix using switching elements.
The range of uses of such an active matrix liquid crystal display device is expanding, and demands for larger screen size, higher definition, and higher aperture ratio are increasing. In addition, it is demanded that the active matrix liquid crystal display device has high reliability and that a production method of the active matrix liquid crystal display device offers high yield and reduces production cost. Simplification of a process is one way for increasing productivity and reducing production cost.
In active matrix liquid crystal display devices, thin film transistors are mainly used as switching elements. In manufacturing thin film transistors, reduction in the number of photolithography steps or simplification of the photolithography step is important for simplification of the whole process. For example, when one photolithography step is added, the following steps are further needed: resist application, prebaking, light exposure, development, postbaking, and the like and, moreover, steps before and after the aforementioned steps, such as film formation, etching, resist removal, cleaning, drying, and the like. Thus, the number of steps is significantly increased only by adding one photolithography step in the manufacturing process. Therefore, many techniques for reducing the number of photolithography steps or simplifying the photolithography step in a manufacturing process have been developed.
Thin film transistors are broadly classified into top-gate thin film transistors, in which a channel formation region is provided below a gate electrode, and bottom-gate thin film transistors, in which a channel formation region is provided above a gate electrode. These thin film transistors are generally manufactured using at least five photomasks.
Many conventional techniques for simplifying the photolithography step use a complicated technique such as backside light exposure, resist reflow, or a lift-off method, which requires a special apparatus in many cases. Using such complicated techniques may cause various problems, thereby leading to reduction in yield. Moreover, there has often been no option but to sacrifice electrical characteristics of thin film transistors.
As typical means for simplifying the photolithography step in a manufacturing process of a thin film transistor, a technique using a multi-tone mask (called a half-tone mask or a gray-tone mask) is widely known. As a technique for reducing the number of manufacturing steps by using a multi-tone mask, Patent Document 1 can be, for example, given.